Liberty Mutual doubles its footprint at 55 Water Street to 120,000 square feet. The deal comes after the landlord has poured millions of dollars into storm-proofing the property.

Slammed by Superstorm Sandy, Manhattan’s largest office building (by square footage) is making a comeback.

Liberty Mutual, the life insurance and financial services company, has signed a 10-year lease for 120,000 square feet at 55 Water Street. The deal doubles Liberty Mutual’s footprint at the property.

The transaction is another sign of the success that landlords of Sandy-stricken downtown buildings are having in convincing tenants to renew and in some cases even expand their offices.

Price has something to do with that. Asking rents at 55 Water Street are in the $40s per square foot, which compares well with Midtown and even Midtown South.

Harry Bridgwood, an executive at New Water Street Corp., which owns and manages the property, said that ownership has invested over $200 million repairing the tower and storm proofing it to avoid damage if future flooding occurs.

The work, which he said includes moving key infrastructure such as electrical systems to the building’s third floor, has eased any lingering concerns about the property’s vulnerability to rising sea levels.

“We’ve invited tenants to see what we’re planning and the work we’re doing so they can be assured,” Mr. Bridgwood said.

55 Water Street, like many office towers on the eastern edge of lower Manhattan, was inundated by flooding during the storm.

Electrical systems were fried, the building’s 80 elevators were paralyzed and its below-grade levels were swamped with millions of gallons of corrosive seawater.

Mr. Bridgwood led a team that has worked around the clock since then, restoring power to the property using temporary generators while rebuilding the electrical systems and making other repairs. He said that the building’s elevators, the majority of which are now working, would be fully operational by next week.

The brokerage company Cresa is representing Liberty Mutual in the deal. CBRE Group Inc. represents 55 Water Street in leasing deals.